A fragile ceasefire took hold in Syria on Thursday, evidence of the pressure Russia, China – and to some extent Iran – could apply to the Assad regime if they wanted to. It was their alarm at the escalating fighting, which Kofi Annan, the joint UN-Arab League envoy, skillfully levered, that produced the first lull in the conflict in months. If the Arab League has no plan B to the six-point Annan plan after the use of UN mandates to protect civilians in Syria was ruled out by Russia and China, it has to be remembered that they have no plan B either. Having bought into the Annan plan, which does not demand regime change, but merely a commitment to an "inclusive Syria-led political process to address the legitimate aspirations and concerns of the Syrian people", Russia and China could do no other than make sure it at least starts.

But a start is only what Thursday's events represent. The other five points calls on Assad to pull the troops out of population centres, make sure aid gets in, release prisoners, allow journalists in and demonstrations to go ahead. On that last point, it will be a brave Syrian who tests this provision today after Friday noon prayers in the mosques. With troops tanks and armoured personnel carriers continuing to patrol the opposition strongholds they have recaptured, Annan lost no time in demanding that Syria's troops pull back from the cities and a UN mission is deployed to observe them doing so. So many attempts at international mediation have failed in the last year, with Assad talking the talk while ordering his snipers to keep on shooting, that it is tempting to dismiss this ceasefire plan as another attempt by Assad to buy time.

But it would be a fatal mistake to dismiss the Annan plan prematurely and argue that because Syria is already burning out of control, a Nato intervention could not precipitate a wider conflict. It could, and it would. Despite the tough words since Monday, when Syrian shelling killed two people in a Turkish refugee camp near the border, Turkey is supremely reluctant to create a humanitarian corridor inside Syria, mindful of what happened with the Kurds the last time this was tried. A regional war on its Syrian border is the last thing Ankara wants or needs. Prime minister Recep Tayyip Erdogan's invocation of Nato's responsibilities under the article 5 mutual aid clause to protect the Turkish border was another expression of how reluctant his country would be to act alone. Add to that brew a fractured opposition, armed rebel groups nowhere near big enough to challenge loyalist forces, the slow rate of defections and the opportunity that jihadis in western Iraq would have to make mayhem just over their border; include Hizbollah and Iran in this mix and the civil war taking place now would be as nothing to the bloodshed that would result from any foreign intervention.

The killing, of course, has not stopped. The Local Co-ordination Committees activist group claimed yesterday that 11 were killed by the Syrian security forces yesterday, six in Homs, four in Idlib and one in Damascus. That is far lower than the average death daily toll of over 100 people. Instead of denigrating the Annan plan, it is in everyone's interests – but particularly Syrian civilians – that it takes hold. It would be an exaggeration to say, as one member of the opposition Syrian National Council claimed, that it will be the beginning of the end of the Syrian regime when it starts to apply the Annan plan. But its full implementation would spell the end of Assad's use of his army to suppress his Arab spring.

Without it, he is more vulnerable to a Yemen-style deal, which removes the figurehead but keeps large parts of the regime in place. Annan has only got a limited time to make this work. His real challenge is to keep Russia and China onside. Annan represents the only common position on Syria that the UN security council has managed to reach. It is a low common denominator, but could yet prove to be the decisive one. If it fails, Russia and China would be forced to return to the security council with a weaker hand.